Preparation method of high-purity boron-carbon spheres for nuclear protection and high-purity boron-carbon spheres

Abstract

The invention belongs to the technical field of ceramic preparation, and particularly relates to a preparation method of high-purity boron-carbon spheres for nuclear protection and the high-purity boron-carbon spheres. The preparation method comprises the following steps: S1, mixing boron carbide powder with graphite powder; s2, pressing the mixed powder into a rod-shaped green body; placing the blank making mold filled with the blank body in a heating container for sintering; s3, the temperature is increased to 1010-1130 DEG C, mixed gas of inert gas and boron trichloride is introduced into the heating container, the gas pressure is controlled to be 1000-1500 Pa, heat preservation is conducted for 5-8 h, and a boron-carbon rod is obtained; s4, processing the boron-carbon rod into a semi-finished boron-carbon ball; and S5, raising the temperature to 1920-2010 DEG C, and carrying out heat preservation on the boron-carbon spheres for 2-3 hours in the atmosphere of inert gas and hydrogen. The problems of high forming difficulty, high manufacturing cost, low production efficiency, low compressive strength and the like of boron-carbon spheres prepared in the prior art are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of ceramic preparation, and particularly relates to a preparation method of high-purity boron carbide spheres for nuclear protection and the high-purity boron carbide spheres. Background Art

[0002] A melt-down accident is one of the most serious potential risks in the nuclear energy field. When a melt-down occurs, the core temperature will rise sharply, causing the nuclear fuel to melt and releasing a large amount of high-intensity radiation such as neutrons and γ-rays, which will cause a devastating disaster to the surrounding environment and the safety of personnel's lives. In the protection system for dealing with melt-down accidents, efficient neutron absorption materials are crucial. Boron carbide materials have attracted much attention due to the high neutron absorption characteristics of boron elements. However, the existing boron carbide materials for nuclear protection are mostly in block or plate form. During a melt-down accident, it is very difficult for the block or plate-shaped boron carbide materials to fill the complex space inside the core. Especially when the boron carbide materials cannot fit around the fuel rods and the key parts at the bottom of the core, it will cause protection loopholes, resulting in a large amount of neutron leakage and accelerating the deterioration of the melt-down accident. Recent studies have shown that making boron carbide materials into spherical shapes can effectively solve the above problems. However, the existing technologies have not been able to solve many problems existing in the forming and firing of boron carbide spheres.

[0003] In terms of forming, mold closing is a conventional technique for preparing ceramic spheres. Since each ceramic sphere must go through a complex process of filling, mold closing, pressurization, and demolding, the production efficiency is low. In addition, in order to ensure the sphericity of the ceramic spheres, the mold needs to have a precise hemispherical cavity. On the one hand, the processing of such a hemispherical cavity is very difficult and the manufacturing cost is very high. On the other hand, due to the extremely high hardness of boron carbide powder, the cavity will soon be worn after frequent mold closing, pressurization, and demolding, seriously shortening the service life of the mold. Moreover, frequent mold replacement will further increase the production cost and cause frequent interruption of the production process.

[0004] In terms of firing, high-temperature sintering is a conventional method for preparing boron carbide ceramics. However, considering that the boron carbide spheres contain a large amount of graphite and a certain porosity, even hot pressing sintering is not sufficient to meet the compressive strength index of the boron carbide spheres, resulting in the boron carbide spheres being extremely prone to damage during transportation, storage, and use. Although adding sintering aids can, to a certain extent, improve the compressive strength of the boron carbide spheres, the actual effect of this approach is not ideal, and introducing sintering aids will inevitably reduce the purity of the boron carbide spheres, thereby affecting their neutron absorption effect. In addition, in order to prevent the boron carbide spheres from collapsing and deforming during high-temperature sintering, the boron carbide sphere blanks must be placed in the hemispherical grooves of the graphite tray. Since only one boron carbide sphere blank can be placed in each pit, it severely restricts the production output of boron carbide spheres per furnace.

[0005] Due to the above-listed series of problems, the application and popularization of boron carbide spheres in the nuclear protection field have always been unable to be realized. Summary of the Invention

[0006] In view of the deficiencies of the above-mentioned existing technologies, the present invention provides a preparation method for high-purity boron carbide spheres for nuclear protection and the high-purity boron carbide spheres, solving the problems of difficult forming, high manufacturing cost, low production efficiency, and low compressive strength existing in the boron carbide spheres prepared by the existing technologies.

[0007] The specific technical solutions are as follows: One of the objectives of the present invention is to provide a preparation method for high-purity boron carbide spheres for nuclear protection, including the following steps: S1. Mixing materials: Mix boron carbide powder and graphite powder evenly to obtain a mixed powder material. S2. Forming a blank: Place the mixed powder material obtained in step S1 into a cylindrical blank-forming mold and press it into a rod-shaped blank; place the blank-forming mold containing the rod-shaped blank into a heating container and wait for sintering. S3. Sintering: Raise the temperature to 1010 - 1130 °C, introduce a mixed gas of inert gas and boron trichloride into the heating container, control the internal air pressure of the heating container at 1000 - 1500 Pa, and keep it warm for 5 - 8 h to obtain a boron carbide rod with elemental boron deposited inside. S4. Processing: Process the boron carbide rod obtained in step S3 into a semi-finished boron carbide sphere. S5. Re-sintering: Raise the temperature to 1920 - 2010 °C, and under the atmosphere of inert gas and hydrogen, keep the boron carbide sphere obtained in step S4 warm for 2 - 3 h to obtain the finished boron carbide sphere.

[0008] The principle of the above preparation method is as follows: In step S3, the elemental boron generated by the high-temperature decomposition of boron trichloride is deposited inside the blank, so that the originally loosely packed boron carbide and graphite particles are firmly bonded together, ensuring the smooth progress of subsequent machining of the boron carbide rod; in step S5, the elemental boron in the semi-finished boron carbide sphere reacts with graphite to generate boron carbide, so that the finished boron carbide sphere contains only two components, boron carbide and graphite, and further improves the strength of the finished boron carbide sphere.

[0009] Furthermore, in step S1: The average particle size of the boron carbide powder is preferably 2 - 5 μm, and the average particle size of the graphite powder is preferably 0.3 - 0.6 μm.

[0010] Furthermore, in step S1: The mass ratio of the boron carbide powder to the graphite powder is preferably 1:(1 - 9).

[0011] Furthermore, in step S2: Axial die pressing is preferably carried out on the mixed powder material in the blank-forming mold with a pressure of 2 - 5 MPa.

[0012] Furthermore, in step S2: The blank-forming mold needs to be made of a high-thermal-conductivity material, with both ends open, preferably a graphite cylinder, especially a graphite circular cylinder.

[0013] Further, in step S2: The heating container is preferably a quartz tube. When the cylindrical blank-making mold is placed in the heating container, the outer wall of the cylindrical blank-making mold should be in close contact with the inner wall of the heating container.

[0014] Further, in step S3: In the mixed gas of inert gas and boron trichloride, the molar ratio of inert gas to boron trichloride is preferably controlled at 1:(0.2 - 0.4).

[0015] Further, in step S3: The flow rate of the mixed gas of inert gas and boron trichloride is preferably controlled at 130 - 160 mL / min.

[0016] Further, in step S3: The gas is introduced from one end (the inlet end) of the heating container, and the other end (the outlet end) is evacuated to maintain the air pressure. The temperature of the blank-making mold is increased from 1010 - 1040 °C at the inlet end to 1100 - 1130 °C at the outlet end. Controlling the temperature of the blank-making mold (i.e., the graphite cylinder) to increase from 1010 - 1040 °C at the inlet end to 1100 - 1130 °C at the outlet end is to make the boron deposition amounts at the front section (the inlet end) and the rear section (the outlet end) of the produced boron carbide rod the same, so as to ensure that the boron carbide balls processed from the front and rear sections of the boron carbide rod have the same composition. Specifically, step S3 can be implemented using the device disclosed in the Chinese patent "A Preparation Method and Device for a High-Purity Porous Boron Carbide Nuclear Control Rod" with the application number CN202510479801.9. The graphite sleeve in this patent application corresponds to the graphite cylinder in the present invention. When using this device, the temperature of the front section of the graphite cylinder is preferably 1010 - 1040 °C, the temperature of the middle section is 1050 - 1080 °C, and the temperature of the rear section is 1100 - 1130 °C.

[0017] Further, in step S3: Preferably during the heat preservation process, for every 1 h increase in time, the flow rate of the mixed gas is lowered by 8 - 10 mL / min. The above technical solution is to ensure that the flow rate of the mixed gas is adapted to the air pressure inside the quartz tube, so as to prevent the front part of the quartz tube from bursting due to excessive air pressure caused by the pores of the blank being blocked.

[0018] Further, in step S3: During the heating-up process, an inert gas is introduced into the heating container. The flow rate of the inert gas is preferably 130 - 160 mL / min, and the internal air pressure of the heating container is preferably maintained at 2000 - 3000 Pa.

[0019] Further, in step S3: after heat preservation, cooling is carried out; during the cooling process, an inert gas with a flow rate of 60 - 90 mL / min is introduced into the heating container, the internal pressure of the heating container is controlled at 5000 - 8000 Pa, the heating of the blank - making mold is stopped to allow it to cool naturally; when the temperature of the blank - making mold drops below 300 °C, the introduction of the inert gas into the heating container is stopped, the internal pressure of the heating container is restored to normal pressure, and when the temperature of the blank - making mold drops below 100 °C, the blank - making mold is taken out to obtain a boron - carbon rod with elemental boron deposited inside.

[0020] Further, in step S4: a ball - making machine is used to process the boron - carbon rod into a sphere, and the cutter head of the ball - making machine is preferably made of boron carbide. The semi - finished boron - carbon rod has components of elemental boron, boron carbide, and graphite; when using a ball - making machine to process the boron - carbon rod into a sphere, since the cutter head of the ball - making machine is made of boron carbide, the powder ground from the boron - carbon rod also only contains elemental boron, boron carbide, and graphite. These powders can be recycled as raw materials, thus avoiding the problem of raw material waste.

[0021] Further, in step S5: the molar ratio of the inert gas to hydrogen is controlled to be 1:(0.8 - 1.1).

[0022] Further, in step S5: during the heating - up process, an inert gas is introduced.

[0023] Specifically, in step S5: the semi - finished boron - carbon spheres are laid flat on a graphite tray, and the graphite tray is placed in a high - temperature furnace for sintering.

[0024] Further, in the above - mentioned steps, the inert gas is preferably argon.

[0025] The second object of the present invention is to provide a high - purity boron - carbon sphere obtained by using the above - mentioned preparation method.

[0026] The beneficial effects of the present invention are as follows: (1) In the chemical vapor deposition process of boron carbide, methane and hydrogen need to be introduced into the mixed gas. These two gases are flammable and explosive, greatly increasing the risk level of the equipment, and the hardness of boron carbide deposited by chemical vapor deposition is extremely high, making the subsequent machining difficulty of the boron - carbon rod increase sharply. The elemental boron deposited in the present invention is in an amorphous state with a lower hardness. Amorphous boron carbide can bond the particles of the blank together without excessively increasing the hardness of the blank, thus ensuring the smooth progress of the subsequent machining of the boron - carbon rod. The finished - product rate of the boron - carbon sphere processing in the present invention can basically reach 100%.

[0027] (2) By using the method of the present invention, the mixed gas can be forced to flow through the pores of the green body, and elemental boron can be evenly deposited on the surfaces of the particles inside the green body. During the high-temperature sintering stage, boron carbide formed by the reaction of elemental boron and graphite can tightly wrap the graphite particles. On the one hand, it significantly improves the compressive strength of the boron-carbon balls, avoiding damage to the boron-carbon balls during transportation, storage, and use. On the other hand, it reduces the shrinkage rate of the green body during high-temperature sintering, thereby enabling the boron-carbon balls to maintain good sphericity.

[0028] (3) By using the method of the present invention, by adjusting the inner diameter of the graphite cylinder, controlling the deposition process parameters of elemental boron, and reasonably controlling the sintering temperature, boron-carbon balls with diameters and porosities meeting actual requirements can be accurately prepared.

[0029] (4) The method of the present invention avoids the problems of low production efficiency in the die closing process and high production costs caused by frequent die replacement. For semi-finished boron-carbon balls, although their strength is not prominent, their supporting force is sufficient to ensure no collapse at high temperatures. Therefore, during high-temperature sintering, it is not necessary to place the semi-finished boron-carbon balls in spherical pits, effectively solving the problem of low loading per furnace and significantly increasing the output of boron-carbon balls per furnace.

[0030] (5) The semi-finished boron-carbon rods are composed of elemental boron, boron carbide, and graphite. When using a ball mill to process the boron-carbon rods into spheres, the cutting head of the ball mill is made of boron carbide, and the powder ground from the boron-carbon rods also contains only elemental boron, boron carbide, and graphite. These powders can be recycled as raw materials, thus avoiding the problem of raw material waste.

[0031] (6) If directly using the high-temperature sintering method to prepare boron-carbon balls, the temperature needs to be raised to 2100 - 2200 °C, and binders and sintering aids need to be added to the raw materials. The high-temperature conditions increase the energy consumption cost. Using binders and sintering aids not only increases the raw material cost but also reduces the purity of the boron-carbon balls. The sintering temperature of the method of the present invention is only 1920 - 2010 °C, and there is no need to add binders and sintering aids to the raw materials, which not only reduces the energy consumption cost but also improves the purity of the boron-carbon balls. Description of the Drawings

[0032] Figure 1 Schematic diagram of the preparation device for high-purity porous boron carbide nuclear control rods used in the specific embodiment of the present invention; Figure 2 Schematic diagram of the reaction chamber structure of the preparation device for high-purity porous boron carbide nuclear control rods used in the specific embodiment of the present invention; Figure 3 Microstructure photograph of the high-purity boron-carbon balls prepared in Example 1 of the present invention; Figure 4 Macroscopic optical photograph of the high-purity boron-carbon balls prepared in Example 1 of the present invention; In the figure: 1 - Flow rate adjustment module; 2 - Temperature control module; 3 - Pressure adjustment module; 4 - Reaction chamber; 5 - Graphite cylinder; 6 - Quartz tube; 7 - Valve. Specific embodiments

[0033] The principles and features of the present invention will be described below in conjunction with examples. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0034] Using the preparation method of the present invention, boron carbide balls with different diameters can be prepared by changing the inner diameter of the graphite cylinder. For the convenience of description, in the specific embodiments, the inner diameter of the graphite cylinder will be fixed at 10 mm and the length at 100 mm, and the diameter of the processed boron carbide balls is 9 ± 0.2 mm.

[0035] The high-purity boron carbide balls in each embodiment are prepared using the device disclosed in Example 1 of the Chinese patent with the application number CN202510479801.9, "Preparation Method and Device for a High-Purity Porous Boron Carbide Nuclear Control Rod". The graphite sleeve in this patent corresponds to the graphite cylinder in the specific embodiments of the present invention, and the atmosphere is adjusted according to the needs of the present invention.

[0036] The device is as Figure 1 、 Figure 2 shown. As Figure 1 is a preparation device for a high-purity porous boron carbide nuclear control rod, including a temperature control module 2 and a flow rate adjustment module 1, a reaction chamber 4, and a pressure adjustment module 3 connected in sequence. A valve 7 is provided between the flow rate adjustment module 1 and the reaction chamber 4, and a valve 7 is provided between the reaction chamber 4 and the pressure adjustment module 3; the flow rate adjustment module 1 is used to control the flow rate of the gas; the pressure adjustment module 3 is used to adjust the gas pressure in the reaction chamber 4.

[0037] As Figure 2 is a schematic structural diagram of the reaction chamber 4. The reaction chamber 4 is composed of a quartz tube 6 and a graphite cylinder 5. The graphite cylinder 5 is located in the quartz tube 6, and the outer wall of the graphite cylinder 5 is in close contact with the inner wall of the quartz tube 6; the graphite cylinder 5 is used to load the green body and is evenly divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section, and a rear section, starting from the air inlet end. The length ratio of the front section, the front middle section, the middle section, the middle rear section, and the rear section is 1:1:1:1:1.

[0038] The temperature control module 2 is connected to the reaction chamber 4. In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, middle section, and rear section of the graphite cylinder 5 respectively. The heating elements adjust the temperature of the graphite cylinder 5 in the reaction chamber 4 by changing the output power; the temperature controller is used to adjust the output power of the heating elements so as to control the temperature, and the thermocouple is used to detect the temperature of the graphite cylinder 5. The temperature controller can be used to collect the temperature data of the thermocouple.

[0039] In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, front-middle section, middle section, middle-rear section, and rear section of the graphite cylinder 5 respectively. The heating elements adjust the temperature of the graphite cylinder 5 in the reaction chamber 4 by changing the output power; the temperature controller is used to adjust the output power of the heating elements so as to control the temperature, and the thermocouple is used to detect the temperature of the graphite cylinder 5. The temperature controller can be used to collect the temperature data of the thermocouple. Example 1

[0040] To prepare high-purity boron carbide spheres for nuclear protection, the method is as follows: S1. Mixing: Boron carbide powder with an average particle size of 2 μm and graphite powder with an average particle size of 0.3 μm are mixed evenly according to a mass ratio of 1:1 to obtain a mixed powder. S2. Forming: The mixed powder is placed in a graphite cylinder, and the mixed powder in the graphite cylinder is axially molded under a pressure of 2 MPa to press the mixed powder into a rod-shaped blank. The graphite cylinder containing the rod-shaped blank is inserted into a quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube. S3. Sintering: Argon with a flow rate of 130 mL / min is introduced from one end of the quartz tube (hereinafter described as the gas inlet end), and the other end of the quartz tube (gas outlet end) is evacuated to keep the internal pressure of the quartz tube at 2000 Pa. The graphite cylinder is heated, and the temperature of the graphite cylinder is controlled to increase from 1010 °C at the front end to 1100 °C at the rear end; among them, the temperature of the front section is 1010 °C, the temperature of the middle section is 1050 °C, and the temperature of the rear section is 1100 °C. Stop introducing argon, and continuously introduce a mixed gas of argon and boron trichloride from the gas inlet end of the quartz tube for heat preservation; control the flow rate of the mixed gas at 130 mL / min, control the molar ratio of argon to boron trichloride at 1:0.2, and control the internal pressure of the quartz tube at 1000 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 8 mL / min. When the heat preservation time reaches 5 h, argon with a flow rate of 90 mL / min is introduced from the gas inlet end of the quartz tube, the internal pressure of the quartz tube is controlled at 8000 Pa, and the heating of the graphite cylinder is stopped to allow it to cool naturally; when the temperature of the graphite cylinder drops below 300 °C, the introduction of argon into the quartz tube is stopped, the internal pressure of the quartz tube is restored to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, the graphite cylinder is taken out to obtain a boron-carbon rod with elemental boron deposited inside; S4. Processing: Use a ball mill equipped with a boron carbide cutting head to process the boron-carbon rod into a semi-finished boron-carbon ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron-carbon ball flat, and use an ultrasonic cleaner to clean and dry the semi-finished boron-carbon ball; S5. Sintering again: Spread the semi-finished boron-carbon balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it up to 1920 °C under normal pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen to be 1:0.8, and keep it warm for 2 h under normal pressure to complete the sintering of the semi-finished boron-carbon balls, and finally obtain finished high-purity boron-carbon balls. Example 2

[0041] To prepare high-purity boron-carbon balls for nuclear protection, the method is as follows: S1. Mixing: Mix boron carbide powder with an average particle size of 3 μm and graphite powder with an average particle size of 0.4 μm evenly according to a mass ratio of 1:4 to obtain a mixed powder; S2. Forming: Place the mixed powder in a graphite cylinder, axially press the mixed powder in the graphite cylinder with a pressure of 3 MPa to press the mixed powder into a rod-shaped blank, and insert the graphite cylinder containing the rod-shaped blank into the quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube; S3. Sintering: Introduce argon with a flow rate of 140 mL / min from one end of the quartz tube (hereinafter described as the gas inlet end), evacuate from the other end (gas outlet end) of the quartz tube to keep the internal pressure of the quartz tube at 2300 Pa, heat the graphite cylinder, and control the temperature of the graphite cylinder from the front end to the rear end to increase from 1020 °C to 1110 °C; among them, the front section temperature is 1020 °C, the middle section temperature is 1060 °C, and the rear section temperature is 1110 °C; Stop introducing argon, introduce a mixed gas of argon and boron trichloride from the gas inlet end of the quartz tube for heat preservation; control the flow rate of the mixed gas at 140 mL / min, control the molar ratio of argon to boron trichloride at 1:0.3, and control the internal pressure of the quartz tube at 1150 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 9 mL / min; After the heat preservation time reaches 6 h, argon with a flow rate of 80 mL / min is introduced from the gas inlet end of the quartz tube, the internal pressure of the quartz tube is controlled at 7000 Pa, and the heating of the graphite cylinder is stopped to allow it to cool naturally; when the temperature of the graphite cylinder drops below 300 °C, the introduction of argon into the quartz tube is stopped, the internal pressure of the quartz tube is restored to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, the graphite cylinder is taken out to obtain a boron-carbon rod with elemental boron deposited inside; S4. Processing: Use a ball mill equipped with a boron carbide cutter head to process the boron-carbon rod into a semi-finished boron-carbon ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron-carbon ball flat, and use an ultrasonic cleaner to clean and dry the semi-finished boron-carbon ball; S5. Sintering again: Lay the semi-finished boron-carbon balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it up to 1950 °C under normal pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen to be 1:0.9, and keep it warm for 2 h under normal pressure to complete the sintering of the semi-finished boron-carbon balls, and finally obtain finished high-purity boron-carbon balls. Example 3

[0042] To prepare high-purity boron-carbon balls for nuclear protection, the method is as follows: S1. Mixing: Mix boron carbide powder with an average particle size of 4 μm and graphite powder with an average particle size of 0.5 μm evenly according to a mass ratio of 1:7 to obtain a mixed powder; S2. Forming a blank: Place the mixed powder in a graphite cylinder, axially press the mixed powder in the graphite cylinder with a pressure of 4 MPa to press the mixed powder into a rod-shaped blank, and insert the graphite cylinder containing the rod-shaped blank into the quartz tube so that the outer wall of the graphite cylinder is closely attached to the inner wall of the quartz tube; S3. Sintering: Introduce argon with a flow rate of 150 mL / min from one end of the quartz tube (hereinafter described as the gas inlet end), evacuate from the other end (gas outlet end) of the quartz tube to keep the internal pressure of the quartz tube at 2700 Pa, heat the graphite cylinder, and control the temperature of the graphite cylinder from the front end to the rear end to increase from 1030 °C to 1120 °C; among them, the front section temperature is 1030 °C, the middle section temperature is 1070 °C, and the rear section temperature is 1120 °C; Stop introducing argon, introduce a mixed gas of argon and boron trichloride from the gas inlet end of the quartz tube, and keep it warm; control the flow rate of the mixed gas at 150 mL / min, control the molar ratio of argon to boron trichloride at 1:0.3, and control the internal pressure of the quartz tube at 1300 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 9 mL / min; When the heat preservation time reaches 7 h, argon with a flow rate of 70 mL / min is introduced from the gas inlet end of the quartz tube, the internal pressure of the quartz tube is controlled at 6000 Pa, and the heating of the graphite cylinder is stopped to allow it to cool naturally; when the temperature of the graphite cylinder drops below 300 °C, the introduction of argon into the quartz tube is stopped, the internal pressure of the quartz tube is restored to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, the graphite cylinder is taken out to obtain a boron carbide rod with elemental boron deposited inside; S4. Processing: Use a ball mill equipped with a boron carbide cutter head to process the boron carbide rod into a semi-finished boron carbide ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron carbide ball smooth, and use an ultrasonic cleaner to clean and dry the semi-finished boron carbide ball; S5. Sintering again: Lay the semi-finished boron carbide balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it up to 1980 °C under normal pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen to be 1:1, and keep it warm for 3 h under normal pressure to complete the sintering of the semi-finished boron carbide balls, and finally obtain finished high-purity boron carbide balls. Example 4

[0043] To prepare high-purity boron carbide balls for nuclear protection, the method is as follows: S1. Mixing: Mix boron carbide powder with an average particle size of 5 μm and graphite powder with an average particle size of 0.6 μm evenly according to a mass ratio of 1:9 to obtain a mixed powder; S2. Forming: Place the mixed powder in a graphite cylinder, axially press the mixed powder in the graphite cylinder with a pressure of 5 MPa, press the mixed powder into a rod-shaped blank, and insert the graphite cylinder containing the rod-shaped blank into the quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube; S3. Sintering: Introduce argon with a flow rate of 160 mL / min from one end of the quartz tube (hereinafter described as the gas inlet end), evacuate from the other end (the gas outlet end) of the quartz tube to keep the internal pressure of the quartz tube at 3000 Pa, heat the graphite cylinder, and control the temperature of the graphite cylinder to increase from 1040 °C to 1130 °C from the front end to the rear end; among them, the front section temperature is 1040 °C, the middle section temperature is 1080 °C, and the rear section temperature is 1130 °C; Stop introducing argon, introduce a mixed gas of argon and boron trichloride from the gas inlet end of the quartz tube, and keep it warm; control the flow rate of the mixed gas at 160 mL / min, control the molar ratio of argon to boron trichloride at 1:0.4, and control the internal pressure of the quartz tube at 1500 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 10 mL / min; After the heat preservation time reaches 8 h, argon with a flow rate of 60 mL / min is introduced from the gas inlet end of the quartz tube, the internal pressure of the quartz tube is controlled at 5000 Pa, and the heating of the graphite cylinder is stopped to allow it to cool naturally; when the temperature of the graphite cylinder drops below 300 °C, the introduction of argon into the quartz tube is stopped, the internal pressure of the quartz tube is restored to atmospheric pressure, and when the temperature of the graphite cylinder drops below 100 °C, the graphite cylinder is taken out to obtain a boron carbide rod with elemental boron deposited inside; S4. Processing: Use a ball mill equipped with a boron carbide cutter head to process the boron carbide rod into a semi-finished boron carbide ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron carbide ball smooth, and use an ultrasonic cleaner to clean and dry the semi-finished boron carbide ball; S5. Re-sintering: Lay the semi-finished boron carbide balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it to 2010 °C under atmospheric pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen to be 1:1.1, and keep it warm for 3 h under atmospheric pressure to complete the sintering of the semi-finished boron carbide balls, and finally obtain finished high-purity boron carbide balls. Comparative Examples 1-4

[0044] Comparative Examples 1-4 are respectively Examples 1-4 in the Chinese invention patent "A High-Purity Porous Boron Carbide Ceramic for Nuclear Control Rods and Its Preparation Method" with the application number CN202411124474.7. Comparative Example 5

[0045] The carbon boron balls are prepared by the mold closing method, and the raw material components refer to Example 1. The method is as follows: S1. Mixing: Mix boron carbide powder with an average particle size of 2 μm and graphite powder with an average particle size of 0.3 μm evenly according to a mass ratio of 1:1 to obtain a mixed powder; S2. Blanking: Use the mold closing method to press the mixed powder into a spherical blank with a diameter of 9 mm, and the molding pressure is 50 MPa; S3. Sintering: Place the spherical blank in a hemispherical groove with a diameter of 9 mm in a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it to 2240 °C and keep it for 3 h under atmospheric pressure to complete the sintering to obtain finished boron carbide balls. Test 1 Performance test of semi-finished boron carbide balls

[0046] The porosity and compressive strength of the semi-finished boron carbide balls obtained in steps S4 of Examples 1-4 are tested. The test results are shown in Table 1. The porosity is measured by the volume density method in the national standard GB / T25995-2010; the compressive strength is measured by the compression test method in the national standard GB / T4740-1999.

[0047] Table 1 Performance test results of semi-finished boron-carbon balls in each example Case Porosity (%) Compressive strength (MPa) Example 1 40.4 15 Example 2 35.5 14 Example 3 31.1 15 Example 4 26.8 14 As shown in Table 1, by adjusting the process parameters, the porosity of the semi-finished boron-carbon balls can be effectively controlled. For Examples 1 to 4, although the porosity of the semi-finished boron-carbon balls decreased from 40.4% to 26.8%, due to the gradual increase in the graphite content in the raw materials, the compressive strength of the semi-finished boron-carbon balls remained at 14 - 15 MPa. This compressive strength range is very suitable for machining using the ball-making machine described in the present invention. Test 2 Morphology analysis

[0048] Perform morphology analysis on the high-purity boron-carbon balls obtained in Example 1, and take microstructural photos and macroscopic optical photos of them.

[0049] Figure 3 is the microstructural photo of the high-purity boron-carbon balls prepared in Example 1. As Figure 1 shown, the high-purity boron-carbon balls have a uniform porous structure. There is good connectivity between the pores, which can ensure that the helium gas generated inside can be discharged smoothly. The particles are firmly bonded together, which helps to improve their mechanical properties.

[0050] Figure 4 is the macroscopic optical photo of the high-purity boron-carbon balls prepared in Example 1. It can be seen that the high-purity boron-carbon balls prepared by the method of the present invention have a very high sphericity, and their surface is very smooth, without defects such as cracks, pits and burrs. Test 3 Performance test and analysis

[0051] Test the porosity, density, compressive strength, total boron-carbon content, carbon content of the products obtained in Examples 1 to 4 and Comparative Examples 1 to 5, and calculate the shrinkage rate after sintering of the boron-carbon balls. The test results are shown in Table 2. The porosity and density are measured by the volume density method and the Archimedes drainage method in the national standard GB / T25995 - 2010 respectively; the compressive strength is measured by the compression test method in the national standard GB / T4740 - 1999; the total boron-carbon content is jointly measured by inductively coupled plasma emission spectrometry and high-frequency combustion infrared absorption method; the carbon content is measured by high-frequency combustion infrared absorption method.

[0052] The shrinkage rate of each example is calculated by the following formula: (Diameter of semi-finished boron-carbon ball - Diameter of finished boron-carbon ball) / Diameter of semi-finished boron-carbon ball × 100%; The shrinkage rate of Comparative Example 5 is calculated by the following formula: (Diameter of spherical blank - Diameter of finished boron-carbon ball) / Diameter of spherical blank × 100%.

[0053] Table 2 Performance Test Results of Each Example and Comparative Example Case Sintering temperature (°C) Porosity (%) <![CDATA[Density (g / cm 3 )]]> Compressive strength (MPa) Total boron and carbon content (wt%) Carbon content (wt%) Shrinkage rate (%) Example 1 1920 31.4 1.64 72 ≥99.9 48.5 8.7 Example 2 1950 27.8 1.66 66 ≥99.9 78.1 7.4 Example 3 1980 24.1 1.73 62 ≥99.9 85.2 6.5 Example 4 2010 20.6 1.81 57 ≥99.9 87.1 5.6 Comparative example 1 2160 30.9 1.74 420 99.8 20.2 - Comparative example 2 2180 35.3 1.63 379 99.8 20.3 - Comparative example 3 2140 27.8 1.82 474 99.8 20.2 - Comparative example 4 2120 23.4 1.93 583 99.8 20.1 - Comparative example 5 2240 31.3 1.64 17 ≥99.9 49.8 12.8 The porosity and density of Examples 1 to 4 are 20.6% to 31.4% and 1.64 to 1.81 g / cm respectively 3 ; the porosity and density of Comparative Examples 1 to 4 are 23.4% to 35.3% and 1.63 to 1.93 g / cm respectively 3 . Generally, the porosity and density ranges of Examples 1 to 4 and Comparative Examples 1 to 4 are basically equivalent

[0054] The compressive strength of Examples 1 to 4 is only 57 to 72 MPa, while the compressive strength of Comparative Examples 1 to 4 is as high as 379 to 583 MPa. Considering that the nuclear control rods prepared in Comparative Examples 1 to 4 are used to regulate the chain reaction rate, and their components are all boron carbide, resulting in a carbon content of only 20.1% to 20.3%, they should have high compressive strength. The boron carbide balls prepared in Examples 1 to 4 are used for the protection of meltdown accidents. Due to the inclusion of a large amount of graphite, the carbon content is as high as 55.7wt% to 87.2wt%, resulting in their unremarkable compressive strength. Nevertheless, the compressive strength of 57 to 72 MPa is sufficient to ensure that the boron carbide balls do not get damaged during transportation, storage, and use

[0055] The total boron-carbon content of Examples 1 to 4 and Comparative Examples 1 to 4 are ≥99.9wt% and 99.8wt% respectively, indicating that Examples 1 to 4 have the advantage of less impurity content. The sintering temperature ranges of Examples 1 to 4 and Comparative Examples 1 to 4 are 1920 to 2010 °C and 2120 to 2160 °C respectively. The lower sintering temperature of Examples 1 to 4 means lower energy consumption costs

[0056] By setting the sintering temperature of Comparative Example 5 to 2240 °C, Comparative Example 5 and Example 1 have the same porosity, density, and total boron-carbon content. The deposition of elemental boron in Example 1 will increase the boron content of the boron carbide ball, making the carbon content of Example 1 slightly lower than that of Comparative Example 5. Although the sintering temperature of Comparative Example 5 is as high as 2240 °C, its compressive strength is only 17 MPa, far lower than 72 MPa of Example 1, and the shrinkage rate of Comparative Example 5 is 12.8%, far higher than 8.7% of Example 1. In addition, the sintering temperature of Comparative Example 5 as high as 2240 °C will greatly increase the sintering cost and seriously reduce the service life of the sintering furnace

[0057] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention

Claims

1. A method for preparing high-purity boron carbon spheres for nuclear protection, characterized in that: The steps include: S1. Mixing: mixing the boron carbide powder and the graphite powder to obtain a mixed powder; S2 greening: The mixed powder obtained in step S1 is placed in a cylindrical greening mold and pressed into a rod-shaped greening body; the greening mold containing the rod-shaped greening body is placed in a heating container to be sintered; S3. Sintering: heating to 1010~1130℃, introducing a mixed gas of inert gas and boron trichloride into the heating container, controlling the internal pressure of the heating container at 1000~1500 Pa, and keeping the temperature for 5~8 hours to obtain a boron carbon rod; S4 processing: the boron carbon rod obtained in step S3 is processed into a semi-finished boron carbon ball; S5. Sintering again: raising the temperature to 1920-2010° C., and keeping the boron carbon balls obtained in step S4 warm for 2-3 h in an atmosphere of inert gas and hydrogen.

2. The preparation method according to claim 1, characterized in that: In step S1: The average particle size of boron carbide powder is 2~5 μm, and the average particle size of graphite powder is 0.3~0.6 μm; The mass ratio of boron carbide powder to graphite powder is 1:(1~9).

3. The preparation method according to claim 1, characterized in that: In step S2: The mixed powder in the blanking mold is axially pressed with a pressure of 2-5 MPa; The cylindrical blank-making mold is a graphite cylinder; The heating container is a quartz tube.

4. The preparation method according to claim 1, characterized in that: In step S3: In the mixed gas of the inert gas and boron trichloride, the molar ratio of the inert gas to the boron trichloride is controlled at 1:(0.2-0.4); The flow rate of the mixed gas of inert gas and boron trichloride is controlled at 130~160 mL / min.

5. The preparation method according to claim 1, characterized in that: In step S3: gas is introduced from one end of the heating container, and the other end is evacuated to maintain the air pressure, and the temperature of the blank-making mold is controlled to increase from 1010~1040℃ to 1100~1130℃ from the air inlet end to the air outlet end.

6. The preparation method according to claim 1, characterized in that: In step S3: during the insulation process, the flow rate of the mixed gas is reduced by 8-10 mL / min for every 1 h increase in time.

7. The preparation method according to claim 1, characterized in that: In step S3: during the heating process, an inert gas is introduced into the heating container, the flow rate of the inert gas is 130-160 mL / min, and the internal pressure of the heating container is maintained at 2000-3000 Pa.

8. The preparation method according to claim 1, characterized in that: In step S4: a ball mill is used to process the boron carbon rod into a ball, and the cutter head of the ball mill is made of boron carbide.

9. The preparation method according to claim 1, characterized in that: In step S5: the molar ratio of the inert gas to the hydrogen is controlled to be 1:(0.8-1.1).

10. A high-purity boron carbon sphere for nuclear protection, characterized in that: Obtained using the preparation method according to any one of claims 1 to 9.

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